Significant advances in the ability to provide optical modulation in a silicon-based platform has been made, as disclosed in U.S. Pat. No. 6,845,198, issued to R. K. Montgomery et al. on Jan. 18, 2005 and assigned to the assignee of the present application. The Montgomery et al. modulator is based on forming a gate region of a first conductivity type to partially overlap a body region of a second conductivity type, with a relatively thin dielectric layer interposed between the contiguous portions of the gate and body regions. The doping in the gate and body regions is controlled to form lightly doped regions above and below the dielectric, thus defining the active region of the device. Advantageously, the optical electric field essentially coincides with the free carrier concentration area in the active device region. The application of a modulation signal thus causes the simultaneous accumulation, depletion or inversion of free carriers on both sides of the dielectric at the same time, resulting in operation at speeds in excess of 10 GHz.
FIG. 1 illustrates one exemplary arrangement of a silicon-based modulator device as disclosed in the Montgomery et al. reference. In this case, a “SISCAP” structure 1 in terms of a doped (i.e., “active”) silicon layer 2 (usually polysilicon) is disposed over a doped portion of a relatively thin (sub-micron) surface layer 3 of a silicon-on-insulator (SOI) wafer 4, this thin surface layer 3 often being referred to in the art as the “SOI layer”. A thin dielectric layer 5 is located between the doped, active polysilicon layer 2 and the doped SOI layer 3, with the layers disposed so that an overlap is formed, as shown in FIG. 1, to define an active region of the device. As mentioned above, free carriers will accumulate and deplete on either side of dielectric layer 5 as a function of voltages applied to SOI layer 3 (VREF3) and/or polysilicon layer 2 (VREF2). The modulation of the free carrier concentration results in changing the effective refractive index in the active region, thus introducing phase modulation of an optical signal propagating along a waveguide formed along the active region (the waveguide being in the direction perpendicular to the paper).
When constructing such a modulator as a pure frequency modulator (i.e., single sideband), a sawtooth ramp waveform, as shown in FIG. 2, is used to provide the modulating signal. In particular, an input signal is used to linearly change the phase from 0 to 2π, and then nearly instantaneously returning to 0 (and then repeating—modulo 2π). This linear phase shift results in a fixed frequency translation: ω0=δφ/δt. However, a problem arises with such modulators that are based on the free carrier effect to provide the desired modulation. That is, the optical absorption/attenuation characteristic of the modulator is a function of the total number of free carriers in the optical path. As a result, the application of a signal to modulate the phase of the optical signal will also affect the amplitude of the optical signal. This is problematic in that the unwanted amplitude modulation introduces error in the output signal. FIG. 3 illustrates the presence of this amplitude modulation and the residual AM modulated signal components in the associated frequency spectrum.
Thus, a need remains in the art to remove, as much as possible, the AM modulation present within an SOI-based electro-optic phase modulator.